LC (Liquid Crystal) or LCD (Liquid Crystal Display) projectors and LC displays, using transmissive LCD panels, are widely employed, both in monochrome (black and white) and color. For example, LCD direct viewing displays, using a single active matrix LCD, are used in lap top and notebook computers and LCD projectors are used to display computer-generated data on a screen. The LCD panel consists generally of a liquid which is held between a pair of transparent plates. Optical transmission, on a light valve pixel-by-pixel basis, is controlled by an active matrix of thin film transistors or diodes deposited on the plates.
Color LCD is preferred to monochrome because it is more life-like and vivid. However, color LCD presents serious problems. First, the color LCDs which are presently commercially available are dim. They may therefore be difficult to see and their colors are not bright or vivid. They generally only transmit about one-third or less of the incident light (luminous flux) compared to a monochome LC panel, due mainly to absorption and spatial distribution of its color filters. If the back light output is raised to make an LCD brighter and to compensate for the poor light transmittal of the LCD panel, the battery life would decrease, which would be a problem in portable computers and other portable devices. In non-portable devices, where power need not be conserved, an increased light output at the lamp may result in greater heat in the device, shorter light source life and a more complex and expensive white light source.
A direct-view display of 2-D images is a pictorial display where the operator looks directly at the light-generating surface and not at a projected image. Typically such direct view displays are presently generated by either a CRT (Cathode Ray Tube) or a direct-view transmitting-type LCD with diffuse back light. Some examples of direct-view displays are as follows: (1) Portable computers--now built with a continuous luminous backlight directly behind the LCD; (2) Avionics displays--CRT's displaying flight and external sensor data; (3) Map displays driven by global position satellite information combined with local vehicle status data, usually CRTs or backlighted LCDs; (4) Instrument displays--generally small CRTs presenting measurement data in highly processed form, such as temperature profile maps from radiometric scanners, spectral data from chemical analysis instruments, maps of defects in semiconductor wafers, etc. Generally the pictorial displays (image displays) are computer generated digital pictures formed by assigning a color/brightness value to each pixel in an X-Y (row-column) matrix of pixels. The usual aspect ratio of such displays is 3:4 vertical to horizontal, and the usual number of row elements is 200 to 1300.
The presently commercially available direct-view displays present a number of limitations and disadvantages. CRTs are long tubes which extend far behind the viewing surface. They require high voltage for operation. Their screen is illuminated by phosphors whose properties limit the brightness of the display. If one sought to increase brightness, the necessary beam current would reduce the life of the CRT. Besides the safety hazards (high voltage, fragility) the CRT is intrinsically inefficient for a single viewer because each activated region of the phosphor radiates forward into a complete hemisphere, and loses some light backwards as well. Most of the emitted light travels outward away from the viewer's line of sight. Home television sets employ large CRTs, up to several feet diagonal, but they are limited in the size of their screens and the brightness of their images.
An alternative to CRT is an LCD panel which is lighted from behind its rear face (backlighting). In a laptop computer, a large luminous area behind the LCD sends light forward to the LCD. Light spreads over a wide angle. For pixels set to "transmit", the light passes through the pixel, emerging with the same angle spread as its incident beam--that is, over a complete hemisphere, +/-90 degrees in any plane. Light at large angles is useless, partly because it does not reach the viewer's eyes and partly because the LCD contrast at large angles is substantially degraded.
The efficiency of the LCD panel is limited by the angle at which the light reaches its rear (entry) face. Only light within a small cone angle around the perpendicular to the LCD panel is useful. The size of a direct-view LCD display is limited by commercially available panels, now about 12 inches diagonal, available in either monochrome or color. In the future, large LCD panels may replace projectors for some applications where a fixed image size is acceptable, for example, home television.
Projection LCD panels are either placed on top of an overhead projector, which acts as the light source and projection apparatus of the system, or are integrated into a system which contains the light source and projection optics.
These projection systems often use expensive, high-power lamps requiring high starting voltage. The lamps have limited lifetime, typically 300 to 1000 hours. They must be kept free from contamination at risk of explosion or fracture, and must be replaced and re-aligned periodically. They operate at extremely high surface temperatures, usually requiring fan air-cooling. A reflector used with such a lamp is generally a metal, low-expansion glass, coated ceramic or quartz, to withstand the local high temperature. All arc lamps generate infrared radiation. In xenon, the infrared (IR) is part of the radiating gas spectrum; and in metal halide lamps the bulb, at extremely high surface temperature (about 600 degrees C.) is a source of longer IR radiation peaking at about 4 microns. The entire bulb envelope and the electrodes radiate to the rest of the system.
The size of the projected image is not limited by the image-forming device (LCD) but by the illuminance required at the image. The projected image must be clearly visible in a room with normal to subdued illumination. Light level at the screen, in turn, is limited by the thermal properties of the image-forming devices. At high light levels, they overheat and cease to function.
The LCD panel used in a projector operates best over a narrow incident angle range (usually +/- a few degrees about its "bias" angle, maybe 7 degrees). Therefore the projector's illuminator must convert the source radiation into a beam with this limited angle spread about the direction of propagation. This will be called "angle population". Any light at greater angles degrades the contrast of the LCD.
LCD panels which project color images have an array of pixels typically 640.times.480 for VGA resolution, 800.times.600 for SVGA resolution or 1024.times.768 for XGA resolution. Other resolutions are also available, such as EWS (Electronic Work Station) at 1280.times.1024. These pixels are controlled typically by an array of tiny transistors (TFT--Thin Film Transistors) or diodes. Each pixel is divided into three rectangular shaped "sub-pixels" (dots); however, each "subpixel" may be considered an independent "pixel" because it is an independently controlled light transmission valve. Consequently, a color LCD having 640.times.480 VGA resolution has (640.times.3).times.480 or 921,600 rectangular shaped sub-pixels (independent light valves) which are called "dots" or "sub-pixels". Each of the three dots has a different color pass-band dye filter passing selectively one of the primary RGB (Red, Green, Blue) colors. A color image is formed by electronically controlling each dot as a shutter (light valve or light modulator), and mixing the RGB colors for each pixel in such a way as to render the desired color on the image.
The RGB color bands employed in electronic imaging are defined under a variety of standards. They are defined in terms of their x,y chromaticity coordinates based on the CIE (Commission International de l'Eclairage) 1931 convention. They can also be described in terms of their spectral distribution curves. There are a number of standards at present. One standard is set by NTSC (National Television System Committee) for the U.S. and North America. A second standard is set by the EBU (European Broadcasting Union) for use in Europe. A third standard is set by SMPTE (Society of Motion Picture And Television Engineers) for use in the electronic projection field. In general terms, the colors are more vibrant (saturated), the closer the x,y coordinates are to the edges of the chromaticity chart. The colors are richer if a certain area of the chart is larger, that area being defined by a triangle connecting the RGB points on the chromaticity chart (color gamut).
Each sub-pixel (dot) of the conventional color LCD panel is dyed one of the RGB colors. As a result of this approach, about two thirds of the white light that falls onto a color dot will be lost. For example, white light falling on a red dot will be allowed to transmit only the red, thus losing the green and blue that are part of the initial white light falling on the dot. Depending on the filter, when the center of the color is selected as the center of the cut-off filter, the light spectra between the pass-band filters is also not transmitted, even though the spectra of the filters overlap. Alternatively, the RGB dots (filters) can be arranged in an array on a separate plate, called a mosaic color filter or stripe color filter. For example, a mosaic filter or LCD may use a triangular pattern (delta) in which each pixel of the LCD is associated with RGB sub-pixels or may use a vertical (orthogonal) stripe pattern in which the vertical stripes (columns) are RGB and are orthogonal to rows of pixels.
A black mask (web) is used between sub-pixels to prevent light leakage and to shield the TFT array from direct light.
Color LCD panels are higher in cost and have greater complexity than monochrome LCD panels. In color, the number of pixels is increased by a factor of three to maintain the same resolution. It is relatively difficult and expensive to produce an accurate and long-lived color filter matrix on an LCD panel.
Increasing the transmissivity of color LCD panels has extremely attractive applications both in projection and direct viewing. For this reason a number of devices have recently been proposed to accomplish this goal.
In U.S. Pat. No. 5,467,206, incorporated by reference, and the article "Compact Spatio-Chromatic Single-LCD Projection Architecture", Loiseaux et al, ASIA Display '95, pgs. 87-70, there is described a phase volume grating (diffractive transmission grating) which separates white light into RGB. That phase volume grating is a thin plastic film, about 10 microns thick, which uses optical duplication of a hologram for its grating, i.e., coherent beam interference recorded in a photopolymer film. The RGB beams are transmitted to a microlens array, with the RGB directed at different angles to each microlens. The microlenses are aligned with an LCD panel having sub-pixels. The microlenses direct the R light to the R sub-pixel, the G light to the G sub-pixel and the B light to the B sub-pixel. The LCD panel controls which of the sub-pixels are off or on (transmitting or non-transmitting), so that the separated RGB are either transmitted or not transmitted.
An alternative projection system is described in the article Hamada et al, "A New Bright Single Panel LC-Projection System Without A Mosaic Color Filter", IDRC (Int. Display Research) '94 Proceedings, 422 (1994), pages 422,423. It uses three dichroic mirrors which are stacked at angles to each other. Those mirrors break up collimated white light into many RGB beams which are directed, at different incident angles, to a single monochrome TFT-LCD (Thin Film Transistor-Liquid Crystal Display). The TFT-LCD has three sub-pixels associated with each lens of a microlens array. Each microlens directs RGB at a different angle toward its appropriate RGB sub-pixel.
The article "High Efficiency Color Filters For LC Projection System Applications" by Ko et al, Eurodisplay Proced. 1996, describes a "micro-dichroic mirror array". A color filter for an LCD panel uses a microprism array in which the microprisms are coated with dichroic mirrors. The microprisms are not direct view prisms.
It has also been suggested, in the literature, that the RGB colors may be projected to the LCD panel in sequence using a color wheel or rotating prism, see P. Jansen et al, "A novel single light valve high brightness HD projector" EURODISPLAY Proceed. 249 (1993). Alternatively, it has been suggested that three diffraction microlens array plates may be used to separate RGB; see Joubert et al, "Holographic elements for LCD projector", OE'LASE '95 SPIE Proceed. 2406 (1995).
Since none of these suggestions are presently commercially available, it is difficult to estimate their light efficiency, complexity and economic feasibility. However, it is believed that they all present various difficulties in arriving at a high light transmittal system which is mechanically and economically feasible.
U.S. Pat. No. 5,506,929, incorporated by reference, discloses a beam collector and light pipe having an adjacent set of microprisms which may be used for backlighting LCDS. The microprisms have a height on the order of one mil (39.37 micrometers).